Graduation Year

2006

Document Type

Dissertation

Degree

Ph.D.

Degree Granting Department

Geology

Major Professor

H. Leonard Vacher, Ph.D.

Keywords

Caves, Eogenetic karst, Carbonate aquifers, Conceptual models, Aquifer heterogeneity, Floridan aquifer

Abstract

Caves, the cornerstone feature of karst aquifers, are little understood in Florida. This dissertation, which analyzes the morphology, elevation, lithologic setting, and hydrology of caves in west-central Florida, demonstrates that the karst of the unconfined Floridan aquifer differs from the paradigm view of karst presented in modern geology textbooks. The differences reflect setting: eogenetic (west-central Florida) vs. telogenetic (conventional). Interpretations about the architecture of cavernous porosity in this dissertation come from detailed surveys (497 stations) of seven air-filled caves.The surveys reveal that solution cavities within the unconfined Floridan aquifer align along NE-SW and NW-SE fractures. The surveys further identify tabular zones of cavernous porosity that extend for tens of meters. Characteristic "plus-sign" passages occur at the intersection of solution-enlarged fractures and the tabular horizons.

The caves, as surveyed, do not connect points of discrete aquifer input to springs. Rather, they are separated by intact bocks of aquifer matrix, ever- narrowing fissures, sediment fills, and breakdown. With an additional 574 spot elevations from 63 previously surveyed air-filled and submerged caves and 526 foot-length cavities encountered in 26 drilled wells, the assembled data reveal that cave passages above and below the watertable of the unconfined Floridan aquifer cluster at similar elevations throughout west-central Florida. At the largest scale, the levels of cavities cut across geologic structure, thus suggesting a water-table origin. The close linkage of the water table and sea level this coastal setting suggests the levels reflect positions of paleosea level. Given that the air-filled caves in west-central Florida reflect higher sea levels,the coastline would have been close when the air-filled caves formed.

The levels organize according to a sea-level datum at elevations of 30 m, 20-22 m, 12-15 m,and 3-5 m. The levels are similar in elevation to nearby terraces evident in GIS and LIDAR topographic data. The terraces correspond to the classic, Quaternary marine terraces of the coastal plain of the southeastern U.S.A. Given that the now-submerged caves reflect lower sea levels, the coastline was far from the caves when they formed. They organize according to a watertable datum at depths of 15 m, 30-40 m, 60-70 m, and > 100 m with some correspondence to marine terrace and paleoshoreline features identified on the sea floor of the west florida shelf using GIS and multibeam bathymetry.

The multigenerational origin of these deeper caves masks the correspondence. Although past water tables are seen to be the first-order control of cave passages regionally, lithology appears to play a significant role at the scale of an individual cave. Approximately 2,000 measurements of matrix permeability from more than 228 m of continuous core from the unconfined Floridan aquifer of west-central Florida reveal a wide-ranging facies-dependent matrix permeability[log k(m2)= -12.9 +/- 1.6, total range]. Solution passages tend to be wider where the matrix permeability is greater. Time-series analysis on measurements of spring discharge from 31 springs and published time series from 28 additional sites reveal key differences between eogenetic and telogenetic karst aquifers, reflecting the difference in matrix permeability of the eogenetic [log k(m2) from -14 to -11] and telogenetic[log k(m2) from -15 to -20] limestones.

For instance, log Q/Qmin flow-duration curves have greater slopes at eogenetic karst springs, a manifestation of lowerratios between the maximum and mean discharge (Qmax/Qmean). Additionally,aquifer inertia as defined on auto correlograms is greater in eogenetic karst than telogenetic karst.Hydrographs of spring flow and water level vary on a seasonal or longertime scale. The localized, convective-style storm events typical of the Florida summer rainy season are not realized as individual peaks in these hydrographs.Apparently, large, widespread, storm events, such as hurricanes in the late summer and fall and frontal systems in the winter and spring, are necessary to produce significant changes in storage. Data from nine pressure transducers in caves and in the aquifer matrix across the unconfined Floridan aquifer all record immediate increases in the water level due to Hurricanes Frances and Jeanne in September of 2004. The increases are simultaneous over large regions.

These changes do not propagate through the aquifer as a pulse like the classic scenario of conduit flow in telogenetic karst aquifers.

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